Ink jet printing is a non-impact method for producing images by the deposition of ink droplets on a substrate (paper, transparent film, fabric, etc.) in response to digital signals. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging and large format printing for outdoor applications such as banners, signages, displays, posters, billboard and bus wraps.
This technique of printing is well suited for printing on a variety of surfaces (receivers) including porous and non-porous surfaces. Porous inkjet receivers have the advantage of large fluid intake and this provides fast printing speed. However, in some applications, such as banners, signages, displays, posters, billboard and bus wraps, these porous receivers suffer from durability issues such as lightfastness, waterfastness, abrasion resistance and weather resistance. In order to overcome these problems, the prints are usually post treated by methods such as lamination, which adds to the cost considerably. Historically, in order to overcome these problems, solvent based inks or UV curable inks were developed to be printed on non-absorbing substrates such as vinyl to achieve the desired durability. U.S. Pat. No. 4,106,027describes such a solvent based ink that is suitable for printing images on non-absorbing surfaces with improved adhesion and durability. EP 0 882 104 B1 describes a UV curable inkjet ink composition for better durability of the print on non-absorbing substrate. A significant environmental, health and safety concern with both of these types of ink is the evaporation of solvent or UV monomer during printing.
U.S. Pat. No. 6,087,416 describes the use of an aqueous based pigment ink for printing on non-absorbing vinyl substrate. That ink contains a grafted copolymer binder that is soluble in the aqueous vehicle but insoluble in water. However, the problem with the ink is that, due to the soluble nature of the binder in the ink vehicle, the ink viscosity is high and the printing reliability is poor. In addition, the printed images are not durable to solvent.
U.S. Pat. No. 4,762,875 describes a printing ink containing pigment and polymer additives for plastics and metal surfaces, however, there is a problem with this invention that the solid content for this ink is high, therefore the viscosity of the ink is too high for inkjet ink applications; in addition, the additives used in this invention is hydrophilic in nature therefore the resulted coatings has poor water resistance.
JP 01262022A discloses an ink jet ink composition containing pigment and a thermal curing agent. There is a problem with this invention in that there are excess of the hydrophilic components remaining in the print, an extra step of “washing” has to be done to remove these materials in order to achieve the image durability.
U.S. patent application Ser. No. 10/083,258 discloses the use of a composition containing a heat sensitive polymer in printing plate applications; However, no use in ink jet inks is disclosed.
Therefore, there is a need for an aqueous based ink jet ink that provides good waterfastness, lightfastness, abrasion resistance, good adhesion to non-absorbing substrates including untreated vinyl, and that can be printed reliably through a piezo or thermal printhead.